Although dental composite resin materials are beginning to live up to expectations as anterior restoratives, they have not been proven to be durable enough to be used routinely in occlusal, load-bearing applications. The main problem appears to be an inadequate ability to resist the degradative forces accompanying the wear process. Recently, the phenomena of wear in composites has been related to the ability of the material to resist internal crack formation and propagation, as well as to its ability to resist subsurface deformation and degradation by solvents and physical stresses. The restorative material's ability to resist internal deformation and fracture depends upon its composition and the physical properties of its components. The goal of this research is to describe the nature of the subsurface microscopic changes in structure which lead to failure of dental composite materials by fracture mechanisms. The fracture toughness, time dependent compressive creep and compressive yield strength of commercial composites and experimental resins will be evaluated, and the results will be correlated to the degree of polymerization (DP) of the resin matrix, as well as to the concentration and size distribution of the particulate reinforcing fillers. Since the properties of the polymeric resin and the filler/matrix interface are suspected to degrade with time, the long-term stability of the materials will also be evaluated by storing specimens in several aqueous solvents for one year prior to testing. The fracture surfaces and the microstructure of the composite subjected to static loading during creep will be examined by scanning electron microscopy. Evidence of microcrack formation in the polymer resin matrix, filler/matrix debonding, redistribution of fillers and other defects will be identified, since these microstructural changes may initiate the fracture processes which cause composite restorative materials to fail during service in the oral environment.